Quantifying the Uncertainty in Modeled Water Drainage and Nutrient Leaching Fluxes in Forest Ecosystems

Nitrate leaching from soil under different forest tree species is related to the vertical distribution of Nitrobacter abundance

Forest tree species and their mineral N uptake strategies can influence the activity and abundance of nitrifying microorganisms in deeper soil layers and subsequent nitrate leaching. However, the role of nitrifier community from the topsoil or deeper soil layers for nitrate leaching below the rooting zone remains uncertain. We evaluated potential nitrification rates and the abundance of ammonia- and nitrite- oxidizers in soil profiles covered by different tree species having (i.e. spruce and Nordmann fir) or not (i.e. Douglas fir, Corsican pine and beech) the Biological Nitrification Inhibition, BNI, capacity. Concurrently, we calculated nitrate fluxes under each tree species by coupling nitrate concentrations in soil solutions with the hydrological model Watfor to simulate water percolation, and analyzed the relationships between nitrate fluxes and nitrifiers characteristics. We observed that nitrification rates under BNI species in the topsoil were lower than those under non-BNI species, and that these changes were associated to strong differences in the abundance of Nitrobacter (500-fold changes between tree species). Nitrification potentials drastically decreased with increasing soil depth and were strongly correlated with the abundance of Nitrobacter, not ammonia oxidizers. Furthermore, by computing weighted mean values of nitrifier activity and abundance, we showed that nitrate fluxes were explained by the abundance of Nitrobacter community across the 0-60 cm soil profile. In this context, the abundance of Nitrobacter community seems an interesting proxy for evaluating water quality at the plot scale, and a promising tool to understand and predict the risk of nitrate leaching from soils in temperate forest ecosystems.

Assessing the recycling of chlorine and its long-lived 36Cl isotope in terrestrial ecosystems through dynamic modeling

It is unclear to what extent chlorine (Cl) and its long-lived isotope ³⁶Cl are recycled in different terrestrial environments in response to time-variable inputs. A new version of a dynamic compartment model was developed to examine the transformation and transfer processes influencing the partitioning and persistence of both Cl and ³⁶Cl in forest ecosystems. The model's performance was evaluated by comparing simulations and field observations of scenarios of stable Cl atmospheric deposition and of global ³⁶Cl fallout. The model reproduced Cl storage in soil reasonably well, despite wide heterogeneity in environmental conditions and atmospheric deposits. Sensitivity analysis confirmed that the natural production of organochlorine in soil plays a major role in Cl build-up and affects long-term Cl dynamics. The timeframe required for the soil organochlorine pool to reach equilibrium in a steady-state system was several thousands of years. Interestingly, root uptake flux, a predominant pathway of the inorganic cycle, was found to affect both inorganic and organic pools in soil, highlighting the importance of plant-soil interactions in Cl dynamics. Model outputs agreed well with local ³⁶Cl measurements, and demonstrated that 90% of the ³⁶Cl found in soil may have come from bomb-test fallout. The pattern of estimated ³⁶Cl/Cl ratios showed that soil ³⁶Cl was not in equilibrium with ³⁶Cl levels in rain input in the post-bomb period. Complete recovery of a natural isotopic ratio in drainage water will need a time close to the residence time of organic ³⁶Cl in soil: i.e., 800 years. A simple dynamic model concept was found to be suitable to illustrate the plant-soil interactions combining both the inorganic and organic Cl cycles acting over different time scales.